The invention relates to a filling transfer apparatus for bone cement with a cylindrical mixing container which is upwardly open and to a filling transfer piston which can be inserted therein, which is releasably connected to the sleeve of a cement injector and which has an opening through which the mixed bone cement can be transferred into the sleeve by a pressing down of the filling transfer piston.
A filling transfer apparatus of this kind is described in detail in EP-A-0 261 182. The latter however has the disadvantage that a filling transfer piston which is wetted by liquid bone cement must first be drawn out of a mixing container and must be disconnected from the actual injector in order to place on an ejection piston corresponding to the inner cross-section of the injector. In this, liquid bone cement should neither be sucked backwards nor spilled nor distributed to the surroundings.
The object of the invention is to improve this situation. This object is satisfied by the characterising features of the independent claim 1 in that an ejection piston, which has an aperture in the region of the opening for the through-flowing cement which can be closed by a plug after the removal of the cement injector, is inserted in the sleeve. This has the advantage that the dynamic seal which is actually distributed over a large periphery and is dimensioned for the pressing out is already installed when the bone cement enters into the injector, and that the smaller remaining aperture need only be statically sealed. In addition, the cross-section of the aperture can be matched to the viscosity of a preferred bone cement in such a manner that a backward flowing is largely prevented. Further improvements result through the subordinate claims 2 to 16. An aperture with a smallest passage area of less than 50 percent of the inner cross-section of the injection sleeve already brings about substantial improvements. In the case of a relatively fluid bone cement a passage area of between 25 and 5 percent of the inner cross-section of the injection sleeve has proved advantageous. In the case of such small apertures of the ejection piston the end of a piston rod of an ejection pistol can be provided with a plug which takes over the static sealing during the pressing out of the bone cement because the pressing force for the ejection piston is transferred via a substantially smaller surface than the piston surface. A frontal and possibly elastic shoulder which can adapt to the inclination of the piston surface of the ejection piston suffices as a static seal and has the advantage that hardened bone cement can be simply removed after use.
A further advantage results when the opening of the filling transfer piston is formed as a nozzle which protrudes into the aperture of the ejection piston. The actual restriction of the bone cement then takes place in the nozzle and only slight taking-along forces arise at the aperture and at the ejection piston during the transfer. When the opening of the nozzle on its outer side lies in contact at the ejection piston, it is prevented that the lower side of the ejection piston is wetted by the bone cement. In the removal of the injection sleeve and the ejection piston then, only a strand of viscous bone cement is still drawn between the nozzle and the aperture. A light pre-pressing between the nozzle and the ejection piston can be achieved when the ejection piston can be fixed in a lower position in the sleeve, for example through a snap connection which is force and form locked. A bulge which extends circumferentially in the cylindrical inner surface of the sleeve and which can only be displaced radially outwardly with a larger axial force at the ejection piston represents a snap connection of this kind.
A further advantage results when the filling transfer piston is blocked in its lower position in such a manner that the sleeve which is filled with cement and the partially closing-off ejection piston can be released from the filling transfer piston in this lower position. If an additional grip part which can still be gripped in the lower position is attached to the filling transfer piston, then a connection between the sleeve and the filling transfer piston can be released by hand in the lowest position of the filling transfer piston. In a rotational lock such as in a bayonet lock it suffices to rotate the grip part and the sleeve oppositely to one another. For a rotational lock, instead of a grip part, a rotational securing through projections and recesses can be attached in the lowest position between the filling transfer piston and the mixing container which transfers a torque from the mixing container to the filling transfer piston which is required for the release. Since with these measures the filling transfer piston need no longer be drawn out in order to release the sleeve, there is also no danger that bone cement is sucked back from the injector into the mixing container.
A grip part has the advantage that relatively large forces can be transmitted to the filling transfer piston for the transfer of the bone cement without a stressing of the connection between the sleeve and the filling transfer piston thereby arising. In addition the cross-section of the aperture and the nozzle can be chosen smaller when large forces can be transmitted to the filling transfer piston.
As concerns the handling, it is advantageous to provide the same kind of lock, for example a bayonet lock, between the sleeve and the filling transfer piston as between the sleeve and the ejection pistol. The sleeve with the bone cement remains in the same hand without a change of grip after the release from the filling transfer piston and can be placed onto the ejection pistol with an opposite movement.
A further functional advantage arises when the sleeve has a conical transition to a distributor tube and when this cone is repeated in the ejection piston and in the aperture as a nozzle. With this measure the outflow resistance to the distributor tube is lowered and the entire volume is pressed out into the distributor tube. In addition a stable guidance of the ejection piston is achieved when the force application of the piston takes place practically above the piston surface and above the sealing surface between the ejection piston and the sleeve. A short guide surface of the ejection piston in the sleeve, which can coincide with the sealing surface, suffices since with this suspension at the piston rod the ejection piston shows no tendency to tilt.
For practical reasons the mixing container can be executed as a transport container for one of the components of the bone cement so that a part which is designed as a throw-away part anyway fulfils a multiple function. A component of the bone cement in powder form can be enclosed in the mixing container for the transport by a cover or a tear-off foil. After the removal of the cover or the tear-off foil further components can be added and mixed by hand with a spatula. Then the filling transfer piston, on which the injector sleeve and the ejection piston were previously delivered in a pre-mounted condition, is placed on. After the mixing the operating personnel need only introduce and press down the filling transfer piston, release the sleeve from the filling transfer piston and place it on the ejector pistol, and this without applying sticky bone cement residues to the surfaces which can be gripped from the outside.